The present invention relates to novel coatings for depositing on microextruded tubes, novel methods of coatings of depositing onto microextruded tubes and novel methods of joining microextruded tubes to headers to complete heat exchanger assemblies.
It is known in the manufacture of heat exchanger assemblies to position an aluminum brazing alloy at the junction between the heat exchanger tubes and the headers so that in the subsequent heat brazing of this junction, a formed fillet or joint is provided between the exchanger tubes and the headers. However, before such aluminum brazing alloys may be utilized to form the fillet, it has been found necessary to either layer the brazing alloy in a sheet form around the junction between the end of the heat exchanger tubes and the headers or to apply the brazing alloy to at least one of the surfaces in a particulate form carried in a liquid vehicle. However, the application of brazing alloys in sheet form is extremely costly, and the application of the brazing alloy in particle form within a liquid vehicle has resulted in uneven and non-uniform coatings.
Additionally, such liquid vehicles for applying the particulate aluminum brazing alloys may become contaminated by the oils which are often utilized in lubricating the heat exchanger tubes to aid in the handling thereof. These oils contaminate the particulate/liquid vehicle mixture thereby resulting in an uneven deposit of such coatings onto the heat exchanger tube and header assembly as well as an improper concentration in the particulate/liquid vehicle mixtures because of the dilution and contamination by the oil into such liquid mixtures.
In all furnace brazing cycles, a flux material is used. The flux material is utilized to remove the oxide ordinarily present on the exposed aluminum metal and other metal surfaces at the location of the joint between the tubes and the headers, as well as to facilitate the flow of molten brazing alloys during the brazing step to fuse and form the fillet or joint between the tube and the header and between tube and fins.
One commercially available flux material is sold under the trademark NOCOLOK, which is a potassium fluoaluminate product available from Alcan International Limited, Montreal, Canada. The NOCOLOK flux material dissolves and otherwise removes the metal oxides on the aluminum surfaces and thereby promotes the flow of brazing alloy about the junction to form the fillet between the heat exchanger tubes and the header and between the tube and fins.
Although the application of aluminum brazing alloys and flux materials has found widespread application in the fillering and joining together of heat exchanger tubes and headers in conventional sized heat exchanger assemblies, when the heat exchanger assemblies include microextruded tubes several problems are encountered when fillering the microextruded tube to the header assembly.
The evolution of the automobile industry has required that significant weight reduction occur with heat exchanger assemblies used in automobile air conditioning units, and has required the reduction in the amount of energy used in such refrigeration units without a reduction in air conditioning or refrigeration capacity. These requirements have dictated a need for more efficient thin walled aluminum tubing which are mounted to header assemblies which will withstand internal pressures developed in the cooling cycle for air conditioning units and refrigeration applications.
The microextruded tube is a thin walled multi-port tube consisting of a generally rectangular shaped tube that is approximately one half to several inches in width and between approximately 0.050 inches a 0.100 inches in thickness. The thickness of the walls of the microextrusion is in the range of approximately 0.010 to 0.020 inches. When microextruded tubes are joined to header assemblies in parallel flow heat type exchangers, several problems are encountered in processing and manufacturing such exchangers using conventional aluminum brazing alloys and flux materials.
For example, it has been found that the application of flux and brazing alloys by using spray and blow-off techniques for depositing the aluminum brazing alloy and the flux material on the junction between the microextrusion tubes and headers in the assembly often times results in an uneven application of the brazing alloy. Also precise deposition of materials is required to insure that there will be sufficient fillet material at the junction of the tube to header to completely fill the gap between the tube and header. In many instances, the deposited aluminum brazing alloy and flux, on the other hand, permits the filler material to enter the inside of the micro-ports of the tube and plug them, thus rendering the tubes useless for heat transfer purposes in the finished heat exchanger assembly.
If the fluxing material also contains silicon or zinc the uneven distribution of the silicon and zinc after brazing causes preferential corrosion sites in the assembly.
If the flux and brazing alloy are deposited in excess, the subsequent brazing furnace operation produces excess chemicals thereby resulting in limited and short cycle life for such brazing furnaces, a costly and expensive problem.
Also, excess oil application to the microextrusion tube for ease of assembly may result in poor adherence of the composition to the tube thereby resulting in an inadequate fillet between the tubes and the header assembly or between tubes and fins.
Additionally, the techniques of spraying zinc, onto the micro-tubes and header assembly results in the wasting of the zinc in the production of the flamed sprayed tubes. The elemental zinc can float free in the air as a result of the flame spraying process which creates a hazardous health condition during the zincating process of applying zinc onto the micro-tubes. Also, any uneven or non-uniform coating of zinc on the tubes causes preferential corrosion sites rendering the finished heat exchanger assembly more prone to corrosion.
Furthermore, lubricating oils are used in many applications to facilitate handling and separation of stacks of the pre-cut microextruded tubes prior to loading into the assembly apparatus to join the micro tubes to the header assembly. This lubricating oil contaminates the flux and final product because such spraying processes becomes saturated with the oil and the entire mix sometimes has to be discarded because the mixture is out of proportional balance.
To overcome the problems associated with joining tubes to headers, it has been suggested to utilize fit-up rods within the headers to control the proper insertion depth of both ends of each micro-tube is equal when the micro-tube is joined to the headers in the heat exchanger assembly. This technique requires the use of special rods inserted into the headers to provide even spacing and expensive header designs. Both techniques are time consuming and expensive.